There is no such thing as a "teacup" dog. "Teacup" is a cute name for a runt, and runts often have more health problems than regular-sized dogs of the breed. Anyway, the yorkie standard prohibits dogs from weighing more than 7 pounds and even a 7 pound dog is tiny.
An Important Message About “Teacup” Yorkies:
Please take this into consideration and make purchasing a healthy pet your top priority, not size. The “novelty” is certainly not worth the pain, heartbreak, or extra expense. Remember, all Yorkies are comparatively small. The most important thing is finding a healthy puppy that will grow into a healthy adult,
especially since you looking at an 11 to 15 year commitment with your Yorkie.
From http://www.ytca.org/faq.html#ATags: Yorkie, puppy